COEFFICIENT OF LINEAR EXPANSION IN DEG. F (a).
„ „ tempered ..... . Mild steel
Wrought iron ...
Cast iron . . .
y j iii
If /<> == rise or fall of temperature, a /° = "expansion or con-
traction for every inch, and
Each inch is increased or decreased by a t° ins.
But strain by mechanical means is b <
Then if at0
yibs ^ E a /°
and total force of expansion on walls, as in Fig. 327 at A B, is
FIbs= Eat a
Necessity of Testing to obtain Unit-strength Con-
stants. — It has been hoped that the cohesive strength of the
various materials might be obtained solely by chemical analysis,
but continued experience seems to shew more and more the
necessity for direct mechanical tests to obtain the strength per
square inch in tension, compression, and shear; hence the use
of testing machines. Certainly it is wise also to refer to chemical
oomposition in stating the quality of a material, in order to know
how far it is safe to heat or otherwise treat the same.
Testing Machines. — One machine generally serves for
tension, compression, and bending experiments, the pulling
shackles being changed to suit No doubt machines will ultimately
be designed to test all combined stresses, and thus verify the
theoretical formulae on which we at present rely. In small machines